1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a network of base stations, base station routers, and/or other wireless access points that are used to provide wireless connectivity to mobile units in geographic areas (or cells) associated with the network. Information may be communicated between the network and the mobile units over the air interface using wireless communication links that typically include multiple channels. The channels include forward link (or downlink) channels that carry signals from the base stations to the mobile units and reverse link (or uplink) channels that carry signals from the mobile units to the base station. The channels may be defined using a time slots, frequencies, scrambling codes or sequences, or any combination thereof. For example, the channels in a Code Division Multiple Access (CDMA) system are defined by modulating signals transmitted on the channels using orthogonal codes or sequences. For another example, the channels in an Orthogonal Frequency Division Multiplexing (OFDM) system are defined using a set of orthogonal frequencies known as tones or subcarriers.
A typical wireless communication link between a base station and the mobile unit includes one or more traffic channels for carrying voice and/or data and one or more overhead channels that include channels for transmitting pilot signals, paging signals, synchronization signals, and the like. The pilot channel carries a pilot signal that is used as the reference against which the mobile unit demodulates all the channels including the traffic channels. The paging channel is used to signal the mobile unit in the event of an incoming call and to respond to calls initiated by the mobile unit. The synchronization channel is used to ensure that the mobile unit is properly time-synchronized with the network. In systems such as the IS-95/3G1X systems, the pilot, paging, and the synchronization channels are broadcast from every sector-carrier (e.g., a base station or a sector served by a base station) to facilitate system acquisition, call setup, and traffic channel transmission. Erroneous reception of any of these channels degrades system coverage and performance.
The forward link coverage of the network depends upon the ability of mobile units to properly demodulate the signaling channels and traffic channels transmitted by base stations in the network. The signal-to-interference-plus-noise ratio (SINR) associated with signals received at the mobile units over each of these channels determines, at least in part, whether the mobile unit will be able to successfully demodulate signals received over these channels. The SINR is a function of the power of the desired signal at the mobile unit (i.e., the signal component) and the sum of the interference power present at the mobile unit and the mobile thermal noise power (i.e., the interference-plus-noise component). In CDMA systems, the interference component in a given frequency band is primarily formed by signals generated by other sectors in the network on the same frequency band. The noise is predominantly thermal noise. Other factors being equal, higher SINRs result in a higher probability of successful demodulation of signals received by the mobile units.
Dedicating more power to the overhead channels in the network increases the SINR for the overhead channels, which may increase the probability of proper reception of the overhead channels that the mobile unit. However, in CDMA systems, increasing the overhead channel power eventually results in marginal gains in coverage/performance because CDMA is limited by its own inter-cell interference. The theoretical point at which an increase in overhead channel power would not result in any improvement in coverage or performance is called the interference limit. As long as the interference limit has not been reached, increasing overhead channel power throughout the network should result in some improvement in network coverage and/or performance. The available sector-carrier power in a CDMA system is shared between the overhead channels and the traffic channels. Consequently, there is a trade-off between overhead channel power and available traffic capacity which results in the potential traffic capacity decreasing as the overhead power increases. The overhead channel power is therefore typically selected to achieve reasonable coverage at full capacity load. Typical settings for the overhead channels allocate 15% of the total available power to pilot signals, 1.5% to synchronization signals, and 5.5% to the paging channels.
FIG. 1 schematically represents a conventional fixed overhead channel power allocation scheme 100. The pilot signal power 105, the paging channel power 110, and the synchronization signal 115 are set to a fixed power percentage with respect to the maximum available cell power 120. The power 125 consumed by the traffic channels varies according to the current traffic load and per-user power requirements, while the overhead channel powers 105, 110, 115 remain fixed. The conventional fixed overhead channel power allocation scheme often allows available cell power to go unused. The unused power in FIG. 1 is the difference between the maximum available power 120 and the total of the overhead and traffic powers 105, 110, 115, 125. Leaving the overhead channel power fixed therefore means that power that could have been used to improve coverage and/or performance goes unused. The wireless communication system therefore does not achieve its optimal levels of coverage and/or performance.